I can give you the idea of what to do, doing problems of the same idea in math, but I don't know formulas for you.

One popped up that might help: d=m/v or v= d/m.

From what I learned in math, you want to find the rate that the 5 kg falls per second (units of length/ second).

Say for instance that it's .1t m/sec t= seconds.

since you don't know seconds, set it off to 3 (because that formula helps show total distance if 't' is given a value).

As in, .1t = 3 and solve for t.

There's probably a more physicish way to do it.

Hope that helps!